Add disease acronyms and update covariates.tsv (#27)

Manually created `download/diseases.tsv` from @gwaygenomics
`tcga_dictionary.tsv` file at https://git.io/vPvTb. See #26.

Added acronym column to `samples.tsv`. In `covariates.tsv` use acronym rather
than full disease name for more manageable column names.

Simplified parts of `4.covariates.ipynb`. Moved n_mutation computation for
samples to `2.TCGA-process.ipynb`.